This invention relates to an electrical servo actuator, and more particularly to an actuator for fuel control valves on combustion engines such as jet engines.
In jet engines, a servo actuator extends a control rod to rotate a crank arm in a direction to shut off a throttle valve at zero-volt input, but the servo cannot control an actuator to absolute zero input. The term, "servo actuator" is here used in the broad sense of an electromechanical device which delivers power to move a control rod. The servo actuator includes an amplifier and position feedback potentiometers. The position feedback signal is compared to a command position signal at the input of the amplifier. Since the amplifier of the servo actuator operates on a difference signal at the input thereof, the position output can only be driven to approach the command position input; it cannot control position to an absolute zero difference. The problem then is to drive the control rod enough for the throttle valve operated by the crank arm to "bottom" (cut off all fuel flow) when a zero-volt command is acted on, i.e., when the command signal input drops to zero.
An attempt to control a servo actuator to a zero-volt position from position feedback potentiometers could be made with some offset signal for the zero input condition, but that may result in repeated servo burn-outs, particularly where the feedback potentiometer exceeds its limits. What is required is to provide a servo actuator with the ability to so drive an actuator rod as to "bottom" an object being controlled by the rod, i.e., to place the object being controlled in an absolute zero position without the risk of position feedback potentiometers exceeding their limits and burning out a servo amplifier.